Pulse oximetry is used to continuously monitor the arterial blood oxygen saturation of adults, pediatrics and neonates in the operating room, recovery room, intensive care units, and increasingly on the general floor. A need exists for pulse oximetry in the delivery room for monitoring the oxygen status of a fetus during labor and delivery, and for monitoring the oxygen status of cardiac patients.
Pulse oximetry has traditionally been used on patient populations where arterial blood oxygen saturation is typically greater than 90%, i.e., more than 90% of the functional hemoglobin in the arterial blood is oxyhemoglobin and less than 10% is reduced hemoglobin. Oxygen saturation in this patient population rarely drops below 70%. When it does drop to such a low value, an unhealthy clinical condition is indicated, and intervention is generally called for. In this situation, a high degree of accuracy in the estimate of saturation is not clinically relevant, as much as is the trend over time.
Conventional two wavelength pulse oximeters emit light from two Light Emitting Diodes (LEDs) into a pulsatile tissue bed and collect the transmitted light with a photodiode positioned on an opposite surface (transmission pulse oximetry), or an adjacent surface (reflectance pulse oximetry). The LEDs and photodetector are housed in a reusable or disposable sensor which connects to the pulse oximeter electronics and display unit. The xe2x80x9cpulsexe2x80x9d in pulse oximetry comes from the time varying amount of arterial blood in the tissue during the cardiac cycle, and the processed signals from the photodetector create the familiar plethysmographic waveform due to the cycling light attenuation. For estimating oxygen saturation, at least one of the two LEDs"" primary wavelength must be chosen at some point in the electromagnetic spectrum where the absorption of oxyhemoglobin (HbO2) differs from the absorption of reduced hemoglobin (Hb). The second of the two LEDs"" wavelength must be at a different point in the spectrum where, additionally, the absorption differences between Hb and HbO2 are different from those at the first wavelength. Commercial pulse oximeters utilize one wavelength in the near red part of the visible spectrum near 660 nanometers (nm), and one in the near infrared part of the spectrum in the range of 880 nm-940 nm (See FIG. 1). As used herein, xe2x80x9credxe2x80x9d wavelengths or xe2x80x9credxe2x80x9d spectrum will refer to the 600-800 nm portion of the electromagnetic spectrum; xe2x80x9cnear redxe2x80x9d, the 600-700 nm portion; xe2x80x9cfar redxe2x80x9d, the 700-800 nm portion; and xe2x80x9cinfraredxe2x80x9d or xe2x80x9cnear infraredxe2x80x9d, the 800-1000 nm portion.
Photocurrents generated within the photodetector are detected and processed for measuring the modulation ratio of the red to infrared signals. This modulation ratio has been observed to correlate well to arterial oxygen saturation as shown in FIG. 2. Pulse oximeters and pulse oximetry sensors are empirically calibrated by measuring the modulation ratio over a range of in vivo measured arterial oxygen saturations (SaO2) on a set of patients, healthy volunteers or animals. The observed correlation is used in an inverse manner to estimate saturation (SpO2) based on the real-time measured value of modulation ratios. (As used herein, SaO2 refers to the in vivo measured functional saturation, while SpO2 is the estimated functional saturation using pulse oximetry.)
The choice of emitter wavelengths used in conventional pulse oximeters is based on several factors including, but not limited to, optimum signal transmission through blood perfused tissues, sensitivity to changes in arterial blood oxygen saturation, and the intensity and availability of commercial LEDs at the desired wavelengths. Traditionally, one of the two wavelengths is chosen from a region of the absorption spectra (FIG. 1) where the extinction coefficient of HbO2 is markedly different from Hb. The region near 660 nm is where the ratio of light absorption due to reduced hemoglobin to that of oxygenated hemoglobin is greatest. High intensity LEDs in the 660 nm region are also readily available. The IR wavelength is typically chosen near 805 nm (the isosbestic point) for numerical convenience, or in the 880-940 nm spectrum where additional sensitivity can be obtained because of the inverse absorption relationship of Hb and HbO2. Unfortunately, pulse oximeters which use LED wavelengths paired from the 660 nm band and 900 nm bands all show diminished accuracy at low oxygen saturations.
According to the invention, more accurate estimates of low arterial oxygen saturation using pulse oximetry are achieved by optimizing a wavelength spectrum of first and second light sources so that the saturation estimates at low saturation values are improved while the saturation estimates at high saturation values are minimally adversely affected as compared to using conventional first and second wavelength spectrums. It has been discovered that calculations at low saturation can be significantly improved if the anticipated or predicted rates of absorption and scattering of the first wavelength spectrum is brought closer to, optimally equal to, the anticipated or predicted rates of absorption and scattering of the second wavelength spectrum than otherwise exists when conventional wavelength spectrum pairs are chosen, such as when conventionally using a first wavelength centered near 660 nm and a second wavelength centered anywhere in the range of 880 nm-940 nm.
The present invention solves a long felt need for a pulse oximeter sensor and system which provides more accurate estimates of arterial oxygen saturation at low oxygen saturations, i.e. saturations equal to or less than 80%, 75%, 70%, 65%, or 60%, than has heretofore existed in the prior art. The sensor and system is particularly useful for estimating arterial saturation of a living fetus during labor where the saturation range of principal importance and interest is generally between 15% and 65%, and is particularly useful for estimating arterial saturation of living cardiac patients who experience significant shunting of venous blood into their arteries in their hearts and hence whose saturation range of principle importance and interest is roughly between 50% and 80%. By contrast, a typical healthy human has a saturation greater than 90%. The invention has utility whenever the saturation range of interest of a living subject, either human or animal, is low.
In addition to providing better estimates of arterial oxygen saturation at low saturations, the sensor, monitor, and system of the invention further provide better and more accurate oxygen saturation estimates when perturbation induced artifacts exist and are associated with the subject being monitored.
When the rates of absorption and scattering by the tissue being probed by the first and second wavelength spectrums are brought closer together for the saturation values of particular interest, improved correspondence and matching of the tissue actually being probed by the first and second wavelengths is achieved, thus drastically reducing errors introduced due to perturbation induced artifacts. For example, when light of one wavelength is absorbed at a rate significantly higher than that of the other wavelength, the light of the other wavelength penetrates significantly further into the tissue. When the tissue being probed is particularly in-homogenous, this difference in penetrations can have a significant adverse impact on the accuracy of the arterial oxygen saturation estimate.
Perturbation induced artifacts include, but are not limited to, any artifact that has a measurable impact on the relative optical properties of the medium being probed.
Perturbation induced artifacts include but are not limited to the following:
(1) variations in the tissue composition being probed by the sensor from subject to subject, i.e., variations in the relative amounts of fat, bone, brain, skin, muscle, arteries, veins, etc.;
(2) variations in the hemoglobin concentration in the tissue being probed, for example caused by vasal dilations or vasal constrictions, and any other physical cause which affects blood perfusion in the tissue being probed; and
(3) variations in the amount of force applied between the sensor and the tissue being probed, thus affecting the amount of blood present in the nearby tissue.
In one embodiment, the present invention provides a fetal pulse oximeter sensor with a light source optimized for the fetal oxygen saturation range and for maximizing the immunity to perturbation induced artifact. Preferably, a far red and an infrared light source are used, with the far red light source having a mean wavelength between 700-790 nm. The infrared light source can have a mean wavelength as in prior art devices used on patients with high saturation, i.e., between 800-1000 nm. As used herein, xe2x80x9chigh saturationxe2x80x9d shall mean an arterial oxygen saturation greater than 70%, preferably greater than 75%, alternatively greater than 80%, optionally greater than 90%.
The fetal sensor of the present invention is further optimized by arranging the spacing between the location the emitted light enters the tissue and the location the detected light exits the tissue to minimize the sensitivity to perturbation induced artifact.
According to a preferred embodiment, electrooptic transducers (e.g., LEDs and photodetectors) are located adjacent to the tissue where the light enters and exits the tissue. According to an alternate embodiment, the optoelectric transducers are located remote from the tissue, for example in the oximeter monitor, and optical fibers interconnect the transducers and the tissue with the tissue being illuminated from an end of a fiber, and light scattered by the tissue being collected by an end of a fiber. Multiple fibers or fiber bundles are preferred.
The present invention recognizes that the typical oxygen saturation value for a fetus is in the range of 5-65%, commonly 15-65%, compared to the 90% and above for a typical patient with normal (high) saturation. In addition, a fetal sensor is subject to increased perturbation induced artifact. Another unique factor in fetal oximetry is that the sensor is typically inserted through the vagina and the precise location where it lands is not known in advance.
The present invention recognizes all of these features unique to fetal oximetry or oximetry for low saturation patients and provides a sensor which optimizes the immunity to perturbation induced artifacts. This optimization is done with a trade-off on the sensitivity to changes in saturation value. This trade-off results in a more reliable calculation that is not obvious to those who practice the prior art methods which attempt to maximize the sensitivity to changes in the saturation value. The improvement in performance that results from these optimizations are applicable to both reflectance and transmission pulse oximetry. An example of a fetal transmission pulse oximetry configuration usable with the present invention is described in U.S. patent application Ser. No. 07/752,168, assigned to the assignee of the present invention, the disclosure of which is incorporated herein by reference. An example of a non-fetal transmission pulse oximetry configuration usable with the present invention is described in U.S. Pat. No. 4,830,014, assigned to the assignee of the present invention, the disclosure of which is incorporated herein by reference.